This invention relates to a process of using "seed crystals" for growing single crystal or highly textured bulk quantities of superconductors such as large domain RBa.sub.2 Cu.sub.3 O.sub.7-.delta. where R is a rare earth, La or Y. More specifically, the invention relates to melt processing high T.sub.c superconductors using seed crystals of lower cost oxide compounds such as, for instance, SrTiO.sub.3, LaAlO.sub.3, or the like, which have high melting or decomposition temperatures above those of the high T.sub.c superconductors but are readily available and of low cost and which are not Tc superconductors themselves.
Large domains of YBa.sub.2 Cu.sub.3 O.sub..delta. materials show high levitation forces because of their size and because of their good flux-pinning strength due to large numbers of small pinning sites. The materials show promise in practical applications requiring levitation or high magnetic fields, such as fly wheels for energy storage, high field magnets, magnetic resonance imaging and efficient electric generators. Previously, SmBa.sub.2 Cu.sub.3 O.sub.x as well as Nd.sub.1+x Ba.sub.2-x Cu.sub.3 O.sub.y have been used as the seeding material during the melt-growth process in making large single domain 123 material (YBa.sub.2 Cu.sub.3 O.sub.x).
More particularly, SmBa.sub.2 Cu.sub.3 O.sub.x (Sm 123) had been used, but this material has a serious defect in that Sm 123 has only a slightly higher melting point than the bulk (YBCO) 123 material and, therefore, the seeds of Sm 123 often melt during the process. This is undesirable, since the purpose of the seeding material is to provide solid particles to facilitate the crystallization of the melted 123 material during the melt-forming process.
It has been found that Nd 123 forms a solid solution in the form of Nd.sub.1+x Ba.sub.2-x Cu.sub.3 O.sub.y and was previously thought to be unsatisfactory as a seed material, but a co-pending patent application Ser. No. 371,931 filed Jan. 12, 1995, assigned to the assignee herein, the disclosure of which is herein incorporated by reference, teaches the use of the Nd 123 material as a seed for preparing bulk superconducting material.
The problem encountered in using either the Sm or Nd 123 materials as seeds is that both of these materials are expensive even though the Nd 123 material is particularly advantageous because of its relatively high melting temperature compared to the melting temperature of bulk superconductor being prepared so that the Nd 123 material does not melt during the high temperature processing of the RBa.sub.2 Cu.sub.3 O.sub.x material.
It has been found that suitable inexpensive seed materials exist which have crystallographic structures and lattice parameters sufficiently close to RBa.sub.2 Cu.sub.3 O.sub.7-.delta. such that large domain material can be prepared by using the cheaper oxides as templates with respect to particular superconductors. The advantage of the present invention is that the crystals of special interest are available commercially and are of relatively low cost.